Transient voltage suppressors (TVS) are devices used to protect integrated circuits from damages caused by over voltage imposed onto the integrated circuit. An integrated circuit is designed to operate over a normal range of voltages. However, in situations such as electrostatic discharge (ESD), electrical fast transients and lightning, an unexpected and an uncontrollable high voltage may accidentally strike onto the circuit. The TVS devices are required to provide protection to circumvent the damages that are likely to occur to the integrated circuits when such over voltage conditions occur. As an increasing number of devices are implemented with integrated circuits that are vulnerable to over voltage damages, demands for TVS protection are also increased. Exemplary applications of TVS can be found in USB power and data line protection, digital video interfaces, high speed Ethernet, Notebook computers, monitors and flat panel displays.
FIG. 1 shows a conventional TVS circuit implemented with a diode array commonly applied for electrostatic discharge (ESD) protection of high bandwidth data buses. The TVS circuit 100 includes a main Zener diode 101 operated with two sets of steering diodes, i.e., the high-side steering diode 103 and the low-side steering diode 105. The high side steering diode 103 connects to the voltage source Vcc and the low side steering diode 105 connects to the ground terminal Gnd with an input/output port I/O connected between the high side and low side steering diodes. The Zener diode 101 has a large size and functions as an avalanche diode from the high voltage terminal, i.e., terminal Vcc to the ground voltage terminal, i.e., terminal Gnd. At a time when a positive voltage strikes the I/O (input/output) terminal, the high side diode provides a forward bias and is clamped by the large Zener diode.
Several device characteristics are desirable in such a TVS. A low TVS clamping voltage is desired to better protect an integrated circuit connected to the TVS. A low clamping voltage ensures that any electrostatic discharge (ESD) will be diverted from the integrated circuit. The device clamping voltage is highly dependent on the breakdown voltage of the Zener/Avalanche diode. Thus, it is also desirable to maintain a low breakdown voltage at the Zener/Avalanche diode in order to improve clamping voltage. The term Zener and Avalanche will be used interchangeably hereinafter to describe a diode that exhibits avalanche breakdown properties. In addition to having a low clamping voltage, and low avalanche diode breakdown voltage, it is also desirable to have an extremely low overall device capacitance. A low device capacitance translates to higher allowable bandwidth and reduction of insertion loss during device operation. It is also desirable to reduce die package size of such TVS devices in order to reduce costs and maintain compatibility with shrinking integrated circuits.
With current TVS devices, there is still a further demand to reduce die size, to reduce device capacitance, and to improve breakdown voltage and clamping voltage characteristics. Therefore, there is a need to provide new and improved device configurations with new structural layouts and manufacturing methods to achieve these goals.
It is within this context that embodiments of the present invention arise.